The invention herein relates to a III-nitride semiconductor LED comprising multiple quantum well layers with different indium concentrations stacked on a substrate.

Prior art III-nitride based blue light emitting structures, such as light diodes (LEDs) and laser diodes (LD), (for the sake of brevity, LEDs and LDs may each be referred to as LEDs herein) are commercially available with peak external quantum efficiency (EQE) exceeding 80%. Operating in green spectral region, the EQE of prior art LEDs drops below half that of blue LEDs. The EQE of III-nitride semiconductor light emitters, very abruptly drops even more so toward the amber and red spectra region. There are two common reasons for the efficiency loss in III-nitride light emitters: (1) a large lattice mismatch between InGaN and GaN layers of the III-nitride light emitting structure where the miscibility becomes prominent with the much higher indium concentration required for longer wavelengths; and, (2) InGaN QWs grown on c-plane polar GaN inevitably suffer from quantum-confined Stark effect (QCSE) resulting from a strong piezoelectric field, which in turn causes a reduction in the radiative recombination rate, especially in the long wavelength regions where higher indium concentration is required.

• US 2009/0250686 A1 describes an LED structure comprising one or more yellow light emitting quantum well layers stacked on one or more blue light emitting quantum well layers with barrier layers between the stacked quantum well layers, the quantum well layers having different Indium concentrations.
• EP 2 610 927 A2 relates to a nitride-based light-emitting device having an active layer interposed between two semiconductor layers, an n-type semiconductor layer and a p-type semiconductor layer. The active layer may have a multi-quantum well structure in which a plurality of quantum well layers and a plurality of barrier layers are stacked one above another. A strain buffer layer is formed under the active layer, i.e. interposed between the n-type semiconductor layer and the active layer.
• US 2006/0049415 A1 relates to multi-colour semiconductor light emitting diode, and comprises a multi-bandgap, multi-quantum well active light emitting region.

Although it is difficult to achieve InGaN-based long wavelengths (amber-to-red at wavelengths greater than 600 nm) in III-nitride light emitting devices, such as LEDs for example, such devices are very desirable in order to realize single chip, solid state lighting and monolithic multi-color light modulating devices. Moreover, the device performance of InGaN-based light emitting structure, such as LEDs and LDs, are less temperature dependent due to the higher bandgap offset than that of other long wavelength light emitting structures such as light emitters based on an AllnGaP material system. In addition, a GaN-based red wavelength emitting LED material structure is beneficially temperature-expansion matched to GaN-based blue and green LEDs, which makes it compatible with GaN-based stacked LED light emitting structures that use wafer bonding to create multi-color solid state light emitters. Thus, InGaN-based long wavelength light emitting structures, such LEDs and LDs, can be superior in many applications.

Within the field of prior art InGaN-based red wavelength light emitters, such as LEDs or LDs, that are grown along the crystalline c-axis, all exhibit "phase separation" (also known to a person skilled in the art as indium segregation) due to poor material quality, see for example R. Zhang et al. in U.S. Pat. App. Publ. 20110237011A1 entitled "Method for forming a GaN-based quantum well LED with red light" and Jong-II Hwang et al in App. Phys. Express 7, 071003 (2015) entitled "Development of InGaN-based red LED grown on (0001) polar surface". This phase separation manifests itself as one or more extra emission peaks in shorter wavelength regions on the spectra, which inevitably reduces color purity as shown in FIGs. 2(b) and (c). Therefore, approaches for increasing indium incorporation while not compromising material quality and device performance are critical to achieve long wavelength emission, amber-to-red, III-nitride based light emitting structures, such as LEDs and LDs. The methods and devices disclosed herein pave the way for high performance, long wavelength III-nitride semiconductor light emitting devices for use in solid state lighting, display systems and many other applications that require greater than 600 nm wavelength solid state light emitters.

The above-mentioned disadvantages of the prior art are overcome by a III-nitride semiconductor LED according to claim 1 and a method of forming a III-nitride semiconductor LED according to claim 7.

Hereinafter, various embodiments will be described with reference to the drawings, wherein the same reference characters denote the same or similar portions throughout the several views.

• FIG. 1 is a cross-sectional view of a portion of an illustrative, but non-limitative, embodiment of a III-nitride semiconductor LED device 10 according to the present invention.
• FIG. 2a is a graph illustrating the EL spectrum of the III-nitride semiconductor LED device 10 of FIG. 1 emitting amber and red light at a current injection of 30 mA. The inset shows the EL spectra of both amber and red LEDs 10 of FIG. 1 within the short wavelength spectral region at 30 and 100mA injection currents.
• FIG. 2b shows the photoluminescence (PL) spectrum in a prior art: U.S. Pat. App. Publ. 20110237011A1.
• FIG. 2c shows the current dependent EL spectra in a prior art: App. Phys. Express 7, 071003 (2015).
• FIG. 3 is a graph illustrating the output power and relative EQE as a function of current for the III-nitride semiconductor LED device 10 of FIG. 1 emitting red light (measurements performed in an on-wafer configuration).
• FIG. 4 is a graph illustrating the peak wavelength shift and full width at half maximum (FWHM) as a function of current for the III-nitride semiconductor LED device 10 of FIG. 1 emitting red light.
• FIG. 5 is a graph illustrating the output power and relative EQE as a function of current for the III-nitride semiconductor LED device 10 of FIG. 1 emitting amber light.
• FIG. 6 is a graph illustrating the peak wavelength shift and FWHM as a function of current for the III-nitride semiconductor LED device 10 of FIG. 1 emitting amber light.

The invention relates to long wavelength light emission III-nitride based semiconductor light emitting structures, such as LEDs LDs, fabricated by means of manipulating the crystalline strain inside the light emitting active region of the structure during the epitaxial growth process. Herein and without limitation, the III-nitride semiconductor light emitter structure of this invention is illustrated within the context of an LED device structure, however a person skilled in the art will recognize how to apply the methods of this invention to the design of other III-nitride semiconductor light emitters including without limitation, LDs.

The present invention discloses an innovative method of fabricating III-nitride based light emitting structures, such as LEDs or LDs that do not suffer from excessive phase separation and thus are capable of emitting amber-to-red light with high spectral purity. Additional advantages and other features of the present invention are set forth in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the present invention. The advantages may be realized and obtained as particularly pointed out in subsequent claims of any application claiming priority to the instant application.

According to one embodiment of this invention, in a III-nitride semiconductor light emitting diode (LED) structure, a first cladding layer is comprised of an n-type III-nitride semiconductor layer. A light emitting active region is formed on the n-type cladding layer that includes indium containing III-nitride layers. A p-type AIGaN is formed on the light emitting active region functioning as electron blocking layer (EBL). Then a second p-type III-nitride cladding layer is formed on the AIGaN layer.

According to a further embodiment of this invention, the light emitting region of the III-nitride light emitter comprises a plurality of multiple quantum wells (MQW) sets separated by one or more intermediate strain compensation layers (hereinafter, referred to as ISCLs) to minimize the crystalline strain. The plurality of MQW sets and/or the ISCLs may be vertically stacked on a surface of a substrate, thereby forming a multilayer stack of MQW sets on the substrate. The multilayer stack may include a 1^st MQW set comprising GaN/InGaN with lower indium concentration, a 2^nd MQW set comprising GaN/InGaN with moderate indium concentration and higher than the indium concentration of the 1^st MQW set and a 3^rd MQW set comprising GaN/InGaN with the highest indium concentration capable of emitting the desired amber-to-red wavelength light. The first two MQW sets are utilized to produce the pre-strain effect on the above III-nitride semiconductor layers of the III-nitride light emitting structure. However, herein more than two MQW sets with variable indium concentration may be used to generate an equivalent pre-strain effect as the two MQW sets do, which is exhibited as a non-limiting example in the present disclosure. In addition, AIGaN layers are inserted as intermediate strain compensation layers (ISCLs) to minimize the total strain in the light emitting region. Through the combination of two (in the present exemplary embodiment) or more pre-strained MQW GaN/InGaN sets and the AIGaN ISCL separating these sets, the emission wavelength of III-nitride LEDs can be extended to amber and red region with high spectral purity and high output power.

In a preferred embodiment of this invention, the light emitting region of the III-nitride light emitting structure of this invention comprises one or more III-nitride barrier layers and lower indium multiple quantum well layers (1^st MQW set), a 1^st ISCL containing one or more III-nitride barrier layers and moderate indium multiple quantum well layers (2^nd MQW set), a 2^nd ISCL, one or more barrier layers and high indium containing multiple quantum well layers emitting amber-to-red light (3^rd MQW set), and a top barrier layer, wherein each of the barrier layers is mainly comprised of GaN; each of the indium containing quantum well layers is comprised of InGaN; and each of the ISCL is comprised of AIGaN.

In a first aspect of the invention, a multilayer III-nitride semiconductor LED is disclosed which comprises a first, second and third set of layered MQW sets. The first set may comprise a first indium concentration. The second set may comprise a second indium concentration that is greater than the first indium concentration. The third set may comprise a third indium concentration that is greater than the second indium concentration. At least one of the first and second sets may be configured to function as a pre-strain layer. A first intermediate strain compensation layer is provided and is comprised of Al_xGa_1-xN (0<x≤1) that is disposed between the first and second sets. A second intermediate strain compensation layer is provided and is comprised of Al_xGa_1-xN (0<x≤1) that is disposed between the second and third sets. The first and second intermediate strain compensation layers may be disposed between two barrier layers and the third set may be configured to emit light having a wavelength ranging from about 600 nm to about 660 nm in the amber-to-red visible range.

In a second aspect of the invention, the first indium concentration may be less than about 17%.

In a third aspect of the invention, the second indium concentration may be greater than about 20%.

In a fourth aspect of the invention, the third indium concentration may be greater than about 30% and may be configured to emit amber-to-red light having a wavelength ranging from about 600 nm to about 660 nm.

In a fifth aspect of the invention, at least one of the intermediate strain compensation layers may comprise Al_xGa_1-xN with x value larger than 0 and less or equal to 1.

In a sixth aspect of the invention, the Al concentration in at least one of the intermediate strain compensation layers may be varied and the first intermediate strain compensation layer may comprise a higher Al concentration than the second intermediate strain compensation layer.

In a seventh aspect of the invention, at least one of the barrier layers may be comprised of GaN, and at least one of the sets may be comprised of InGaN, and at least one of the intermediate strain compensation layers is comprised of Al_xGa_1-xN (0<x≤1).

In an eighth aspect of the invention, a method of manufacturing a III-nitride semiconductor LED which emits amber-to-red light, is disclosed comprising steps of defining a first barrier layer, defining a first set of MQW sets comprising a first indium concentration on the first barrier layer, defining a second barrier layer on the first set, defining a first intermediate strain compensation layer comprised of Al_xGa_1-xN (0<x≤1) on the second barrier layer, defining a third barrier layer on the first intermediate-strain compensation layer, defining a second set of MQW sets comprising a second indium concentration that is greater than the first indium concentration on the third barrier layer, defining a fourth barrier layer on the second set, defining a second intermediate-strain compensation layer comprised of Al_xGa_1-xN (0<x≤1) on the fourth barrier layer, defining a fifth barrier layer on the second intermediate-strain compensation layer, and, defining a third set of MQW sets comprising a third indium concentration that is greater than the second indium concentration configured to emit light having a wavelength ranging from about 600 nm to about 660 nm.

In a ninth aspect of the invention, at least one of the barrier layers made by the method may be comprised of GaN, at least one of the sets is comprised of InGaN, the first intermediate-strain compensation layer may comprise a higher Al concentration than the second intermediate-strain compensation layer, the first set may comprise indium concentration not greater than about 17%, the second set may comprises an indium concentration greater than about 20% and the third set may comprise an indium concentration greater than about 30% for emitting amber-to-red light.

The present invention provides an innovative LED device and method for manufacturing III-nitride solid state light emitting structures, such as LEDs or LDs, that emit amber-to-red wavelength light. The III-nitride solid state light emitter is epitaxially grown using the methods of this invention and thus uniquely achieves amber and red wavelength light emission that exhibit high output power and high spectral purity and is free of the phase separation that plagues prior art III-nitride light emitting structures at long wavelengths.

The III-nitride semiconductor amber-to-red wavelength light emitter of this invention is described herein within the context of the LED device structure illustrated in FIG. 1 as an illustrative exemplary embodiment of the present invention. FIG. 1 shows a cross sectional view of a portion of an illustrative embodiment of the amber-to-red III-nitride LED 10 according to the present invention. The amber-to-red III-nitride LED device 10 can be manufactured by conventional epitaxial methods, for example metalorganic vapor phase epitaxy (MOVPE), also known as metalorganic chemical vapor deposition (MOCVD). As shown in FIG. 1, a GaN buffer layer 2 of approximately 2 µm is grown on a substrate 1, for example a sapphire substrate, with a nucleation layer (not shown) interposed in between. An approximately 3 µm thick n-type GaN cladding layer 3 doped with Si is grown on the GaN buffer layer 2. The illustrative embodiment of the amber-to-red III-nitride LED 10 can be epitaxially grown in either a polar, semi-polar or non-polar crystallographic orientation using a suitable substrate 1 crystallographic orientation.

Referring to FIG. 1, the III-nitride LED structure comprises a light emitting region or a multiple quantum well (MQW) active region 4 which comprises indium-containing III-nitride semiconductor layers which are grown on the n-type GaN cladding layer 3. As illustrated in FIG. 1, the active region 4 may comprise three types of MQW sets with indium concentration progressively increased from bottom 4_A MQW set to the top MQW set 4_H. Each MQW set includes one or more 2-3 nm thick (it could be thicker depending on the crystallographic orientation) InGaN quantum well layers (4_A2, ..., 4_E2, ..., 4_H2 ...) and one or more 5-20 nm thick barrier layers (4_A1, ..., 4_B, 4_E1, ..., 4_F, 4_H1, ..., 4_I) mainly comprised of GaN, which are alternatively stacked one on another with each InGaN quantum well layer being sandwiched between two barrier layers. Accordingly, as shown in FIG. 1, the MQW sets 4_A, 4_E and 4_H may be vertically stacked to produce a multilayer stack of MQW sets 4. The III-nitride based barrier layers (4_A1, ..., 4_B, 4_E1, ..., 4_F, 4_H1, ..., 4_I) may include additional amounts of indium and/or aluminum as needed in order to adjust the quantum confinement levels of their respective quantum well layers (4_A2, ..., 4_E2, ..., 4_H2 ...). The indium concentration for the 1^st MQW set 4_A and the 2^nd MQW set 4_E, are preferably in the range of 7-13% and 20-25%; respectively, in the illustrated example. The first two MQW sets 4_A and 4_E produce a pre-strain effect for the top MQW set 4_H, which may contain indium concentrations larger than 30% as preferred for amber-to-red light emission.

In one embodiment of this invention, high indium concentration is achieved without incurring phase separation by careful control of the strain of the high indium-containing quantum well layers (4_H2, ...) of the top MQW set 4_H of the III-nitride amber-to-red light emitting structure of FIG. 1. In this embodiment, introducing the lower two MQW sets 4_A and 4_E with indium concentrations progressively elevated generates a pre-strain effect on the barrier layers thus facilitating high indium intake within the upper most MQW set 4_H. It is emphasized that the inclusion of only one or even two MQW sets with low indium concentration within the III-nitride light emitting structure may not produce a fully functional III-nitride semiconductor LED emitting at amber-to-red region with high spectral purity and high output power. Therefore, according to another embodiment of this invention and as shown in FIG. 1, the intermediate strain compensation layers (ISCLs) 4_C and 4_G are inserted in between each two successive MQW sets.

The ISCLs are sandwiched between the barrier layers of each two successive MQW sets and are preferably composed of Al_xGa_1-xN (0<x≤1) alloys with varied Al concentration x in the range of about 17% to about 25%. The thickness and Al concentration in the ISCLs 4_C and 4_G are preferably different. In one embodiment of this invention, the Al concentration x in the lower ISCL 4_C may be higher than that of the upper ISCL 4_G in order to prevent layers from cracking and to avoid excessive series resistance of LED device 10 because of those layers. In another embodiment of this invention, the Al concentration x in the lower ISCLs 4_C may be lower than that of the upper ISCL 4_G in order to suppress carrier injection into the lower two low-indium concentration MQW sets 4_A and 4_E in favor of promoting higher levels of carrier injection into the high-indium concentration top MQW set 4_H thereby increasing the ratio of LED device light emission from the top MQW set 4_H and thus reducing the full width at half maximum (FWHM) of the amber-to-red light emission of the III-nitride semiconductor light emitter of this invention. In general, the Al concentration is varied ISCL to ISCL, though alternatively, the Al concentration in an individual intermediate strain compensation layer may be varied within that intermediate strain compensation layer, in discrete steps (graded) within the layer, or as a continuous variation in the respective layer.

A person skilled in the art will recognize the use of the aforementioned methods of this invention to select the most appropriate thickness and Al concentration in the ISCLs 4_C and 4_G depending upon the target performance parameters of the amber-to-red light emission of the III-nitride semiconductor light emitter.

Referring back to FIG. 1, overlying MQW active regions 4 of LED device 10 are p-type layers having a combined thickness of about 200 nm, respectively and including a Mg-doped AIGaN electron blocking layer (EBL) 5, a Mg-doped GaN cladding layer 6 and Mg-doped GaN contact layer 7. The atomic concentration of Mg in the p-type AIGaN and GaN cladding layer are preferably in the range from 1E19 cm^-3 to 1E20 cm^-3, for example. The Mg atomic concentration in p-type GaN contact layer is preferably in the range of 1E21 cm^-3, for example.

As in prior art III-nitride light emitters, the III-nitride semiconductor amber-to-red light emitting device of this invention is formed by first depositing on the top side p-GaN layer 7 of the epitaxially processed wafer, an ohmic contact metal stack to form the p-side electrode 8 then etching lateral trenches to expose n-GaN layer 3, then depositing within the etched trenches, an ohmic contact metal stack to form the n-side electrode 9. The epitaxially processed wafer is then diced to form individual LED chips, each having substantially the cross-section illustrated in FIG. 1, which are subsequently packaged and wire-bonded to the respective p-electrodes and n-electrodes to form the LED device of the invention. For lab testing, a p-side electrode (or ohmic contact metal stack) 8, for example, and indium ball (for simplicity in quick-test) is formed on the p-type GaN contact layer 7.

Additionally, one lateral portion of the device 10 is etched out from the p-type GaN contact layer 7 to a portion of the n-type GaN clad layer 3. Then an n-side electrode (or ohmic contact metal stack) 9, for example, indium ball (for simplicity in quick-test) is formed on the exposed portion of n-type GaN clad layer.

Under electrical current injection through the positive p-side electrode 8 to the n-side electrode 9, through the electron and hole recombination process, visible light is emitted from active region 4. The III-nitride semiconductor LED device 10 that is epitaxially grown in accordance with the methods of this invention is preferably configured such that only the top MQW set 4_H emits light while the bottom two MQW sets 4_A and 4_E function mainly as pre-strain layers. A III-nitride semiconductor LED device 10 with ∼1 mm² area emitting light within the amber-to-red (amber-red) visible spectrum region under electric current injection of about 30 mA emits strong amber-red emission.

FIG. 2a is a graph illustrating the typical EL spectra for amber-red LEDs 10 of FIG. 1 driven at about 30 mA. The inset shows the EL spectra of both amber and red exemplary embodiments of the III-nitride LED 10 of FIG. 1 of this invention at the short wavelength spectral region at 30 and 100mA injection currents. As seen in the inset of FIG. 2(a), there are no additional peaks at shorter wavelength regions even under a higher driving current of 100 mA, suggesting no phase separation in the LEDs of the invention. On the contrary, as shown in FIGs. 2b and 2c which illustrate prior art LED performance, phase separation-induced additional emission peaks (∼440 nm) and substantially wider FWHM emissions are readily observed in prior art LEDs: U.S. Pat. Application 20110237011A1 and App. Phys. Express 7, 071003 (2015); respectively.

FIG. 3 through FIG. 6 demonstrate the performance of the amber-red III-nitride light emitter 10 of this invention; FIG. 3 and FIG. 4 demonstrating the performance of one exemplary embodiment of this invention whereby the device 10 active region 4 was designed for red (∼ 625 nm) light emission (∼ 45% indium concentration), while FIG. 5 and FIG. 6 demonstrate the performance of another exemplary embodiment of this invention with the device 10 active region structure designed for amber (∼ 615 nm) light emission (∼ 40% indium concentration).

FIG. 3 is a graph illustrating the measured current dependence of output power and relative EQE for a red emission III-nitride LED device 10 of this invention having its active region 4 designed to peak at red (∼ 625 nm) light emission. All measurements are performed in an on-wafer test configuration with indium balls as both p-side and n-side electrodes 8 and 9, respectively. No special surface treatments and/or cooling units were used to conduct the illustrated testing. The light was collected by an integrating sphere placed underneath LED 10 therefore not all light was collected. As shown in FIG. 3, the output power increases with applied current following a power rule as is observed in other III-nitride semiconductor based LEDs. At 30 mA, the output power of red III-nitride semiconductor LED 10 reaches about 211 µW. The relative EQE of the red III-nitride semiconductor LED 10 peaks at about 35% at around 11 mA and then starts to drop monotonically with increasing current. At 100 mA, the relative EQE was reduced by about 26% compared to the peak value. The true mechanism for the EQE reduction with increasing currents is still not well understood in LED design community. There are basically two explanations for this phenomena: (1) Auger recombination; and (2) electron leakage due to insufficient hole transport which detailed description is beyond the scope of this disclosure.

FIG. 4 is a graph illustrating an example of the measured emission peak wavelength shift and FWHM with current for the red III-nitride LED 10 of FIG. 1. The emission peak wavelength shows a blue shift with increasing injection current. This behavior is a characteristic of the carrier screening of the piezoelectric field in III-nitride based semiconductor LEDs. The band-filling effect is also another cause for the blue shift. The emission peak wavelength shifts at low current due to strong band-filling but gradually saturates to 617 nm at 100 mA. At 30 mA, the emission peak wavelength is 625 nm (red) and the FWHM reaches a minimum value of 49 nm at the same current level for a device with ∼1 mm² area. To the best of the Applicant's knowledge, this illustrates the best red emission performance from III-nitride light emitter achieved to date.

FIG. 5 is a graph illustrating the current dependence of output power and relative EQE of the III-nitride semiconductor LED device 10 of FIG. 1 of this invention having its active region 4 designed to peak at amber light emission. In a similar behavior as that of the red III-nitride semiconductor LED of the previous exemplary embodiment, for a device with ∼1 mm² area, the relative EQE of the amber III-nitride semiconductor LED peaks to 45% at around 9 mA and reduces to about 33% at 100 mA. The measured output power for amber III-nitride LED was about 266 µW at 30 mA.

FIG. 6 is a graph illustrating an example of the measured variations in emission peak wavelength and FWHM with applied current for the amber III-nitride semiconductor LED 10 of FIG. 1. Following the similar trend as red III-nitride LED, the wavelength shifts at low current but gradually saturated to 599 nm at 100 mA. At 30 mA, the peak wavelength is 617 nm (amber) with a FWHM 54 nm.

As described in the preceding paragraphs, III-nitride semiconductor LEDs emitting amber-to-red light with high output power and high spectra purity, according to the present invention, can be readily manufactured by careful control on the material strain. The epitaxial growth methods are compatible with the techniques to obtain III-V compound semiconductors. Suitable epitaxial deposition techniques for use in practicing the present disclosure include, but are not limited to, MOVPE, molecular beam epitaxy (MBE), and hydride vapor phase epitaxy (HVPE). The III-nitride-based semiconductor layers can, for instance, be comprised of Al_xGa_yIn_1-x-yN, where 0≤x≤1 and 0≤y≤1. The plane orientation of the substrate 1 could be either c-plane, semi-polar and non-polar crystalline planes.

Moreover, the LED device 10 according to the present disclosure can be readily fabricated utilizing conventional III-V compound semiconductor manufacturing methodologies and technologies.

While the forgoing disclosure has been described in a way of example, it is to be understood that the disclosure is not limited to thereto. The scope of the present invention is defined by the appended claims.